Sensing of angular velocity is frequently performed using an inertial sensor. Inertial angular velocity sensors broadly function by driving the sensor into a first motion and measuring a second motion of the sensor that is responsive to both the first motion and the angular velocity to be sensed.
Frequently, a mass (usually referred to as a proof mass) within the sensor is driven into oscillation by an actuator. Rotation of the sensor imparts a Coriolis force to the oscillating mass that is proportional to the angular velocity (or rotation rate), and depends on the orientation of the angular velocity vector with respect to the velocity vector of the proof mass. The Coriolis force, the angular velocity vector and the mass velocity vector are mutually orthogonal. For example, a proof mass moving in an X direction within a sensor rotating about a Y axis experiences a Z directed Coriolis force. Similarly, a proof mass moving in an X direction within a sensor rotating about a Z axis experiences a Y directed Coriolis force. Finally, a proof mass moving in an X direction within a sensor rotating about the X axis experiences no Coriolis force. Coriolis forces imparted to the proof mass are usually sensed indirectly by measuring motions within the sensor that are responsive to the Coriolis forces.
Recently, the development of micromachining technology (also known as MEMS technology) has led to the development of various MEMS angular velocity inertial sensors. MEMS technology is basically a planar technology, where suitable MEMS actuators for driving in-plane motion tend to differ significantly from suitable MEMS actuators for driving out-of-plane motion. Similarly, suitable MEMS sensors for measuring in-plane motion responsive to Coriolis forces tend to differ significantly from suitable MEMS sensors for measuring out-of-plane motion responsive to Coriolis forces. These differences are both structural differences and performance differences.
An in-plane MEMS angular velocity sensor must either drive an out-of-plane motion or sense an out-of-plane motion in order to detect an in-plane angular velocity component, due to the orthogonality of mass velocity, angular velocity and Coriolis force discussed above. In contrast, an out-of-plane MEMS angular velocity sensor can drive and sense two orthogonal in-plane motions in order to detect an out-of-plane angular velocity component. Due to the planar nature of MEMS technology, in-plane MEMS sensors and out-of-plane MEMS sensors tend to differ significantly.
Some known in-plane MEMS angular velocity sensors have two proof masses driven into oscillation. For example, U.S. Pat. No. 6,481,283 to Cardarelli teaches an in-plane MEMS sensor. In the coordinates of Cardarelli, the device plane is the YZ plane. In a first embodiment, Cardarelli teaches two masses dithered in the +/−Y direction (i.e., in-plane). Angular velocity about a Z axis leads to X directed Coriolis forces on the two masses. The two masses are attached to a gimbal rotatable about the Z axis such that X directed forces on the masses provide Z directed torques on the gimbal. The two masses are dithered to have oppositely directed velocities, so the two Coriolis forces provides a net torque on the gimbal about the Z axis. Motion of the gimbal about the Z axis is sensed.
In a second embodiment, Cardarelli teaches two masses dithered in the +/−X direction (i.e., out-of-plane). Angular velocity about a Z axis leads to Y directed Coriolis forces on the two masses. The two masses are attached to a gimbal rotatable about the Z axis such that Y directed forces on the masses provide Z directed torques on the gimbal. The two masses are dithered to have oppositely directed velocities, so the two Coriolis forces provides a net torque on the gimbal about the Z axis. Motion of the gimbal about the Z axis is sensed.
Another known in-plane MEMS angular velocity sensor having two proof masses driven into oscillation is taught in U.S. Pat. No. 6,508,122 to McCall et al. McCall et al. teach an in-plane MEMS sensor having two unconnected masses that are laterally disposed in the device plane and dithered out of phase with respect to each other in this plane direction. For definiteness, let the device plane be the XY plane, and let the dither be in the X direction. The masses oscillate in the Z direction when the sensor is rotated about the Y axis, due to Z-directed Coriolis forces. The Z directed oscillation of the masses is sensed.
The approaches of both Cardarelli and McCall et al. are motivated by a desire to reject “common mode” interference from the measurement of angular velocity. For example, an angular velocity sensor having a single proof mass can register an incorrect reading if subjected to a linear acceleration in the same direction as the Coriolis force to be sensed. With two masses, various arrangements are possible, including those mentioned above, that respond to Coriolis forces but generally do not respond to linear acceleration in the same direction as the Coriolis forces. Typically, such arrangements depend on driving the two masses so that their velocities are always equal and opposite. Any deviation from a condition of equal and opposite velocities is disadvantageous, since such deviation reduces the desired response to the Coriolis forces, and increases the undesired response to linear acceleration.
However, in practice it is not straightforward to drive two masses with equal and opposite velocities. For example, two nominally identical and identically mounted masses can differ in practice so that actuating these two masses with the same actuation provides velocities which are not equal and opposite. Actuators tend to vary in effectiveness as well, so even if two masses were identical and identically mounted, variation in the actuators connected to the two masses could again provide mass velocities which are not equal and opposite. Similarly, circuitry connected to actuators may not be identical, etc. As a result, known two mass in-plane angular velocity sensors have not fully realized the common mode rejection promised by two mass configurations.